It is standard practice to electroplate gold from solutions of potassium aurocyanide containing weak organic acids and their salts to buffer the plating solution at a selected pH. The baths may be operated when used for barrel plating at a limiting current density of about 1-2 amperes per square foot. Although satisfactory deposits are obtained under these conditions, the acidity of an acid bath may attack the basis metal and the plating bath may be economically too slow for certain applications. Increasing the current density to increase the speed of deposition results in foxy scum or burned deposits that are not acceptable or structurally sound.
Besides the deposition in pure form, gold is frequently plated from baths containing cobalt, nickel, zinc, copper and indium to form an alloy. The metallic additives are electrodeposited with the gold to produce alloys which have useful and valuable properties. To control the amount of alloying metal in the deposit, chelating agents such as glycine, diethyl glycine, nitrilotriacetic acid, ethylene diamine diacetic acid, imino diacetic acid, ethylene diamine tetracetic acid, diethylene triamine pentaacetic acid, cyclohexane diamine tetraacetic acid and related compounds have been used to control the concentration of the alloying metal ion in the plating bath and, therefore, regulating the amount of metal which is alloyed with the gold deposit. These compounds, known as amino carboxylic acid chelating agents, because of their affinity for the alloying metal ion, function as metal ion buffers in controlling the concentration of free metal ion in the plating bath which in turn controls the amount of metal alloyed with the gold deposit.
Other types of chelating agents have been used in the electroplating of metals. In U.S. Pat. No. 3,475,293, organic phosphonic acids have described for use in ferrous and non-ferrous metal plating baths. The use of organic phosphonic acid in gold plating baths has been disclosed in British Pat. No. 1,198,527. The cited patent ascribes two functions to the phosphonic acid, (1) as a metal chelating agent and (2) as a brightening agent.
Studies on the chelating agents of the aminoethylene phosphonic acid types indicate that their affinities for metal ions such as cobalt, nickel, iron, zinc and copper are less than those of the corresponding aminomethylenecarboxylic acid. The affinity of a chelating agent for a metal ion is defined in this context as a stability constant. The measurement of these values is described in "The Determination of Stability Constants" by F. J. C. Rossati and H. Rossatti, McGraw-Hill Book Company, 1961. Stability constants for aminomethylene phosphonic acid are described by S. Westerback, K. S. Rajan and A. E. Martell in the J. Am. Chem. Soc. 87,2567 (1965).
The synthesis of chelating agents containing both aminomethylene carboxylic acids and aminomethylene phosphonic acids has been described by K. S. Rajan, I. Murase and A. E. Martell, J.Am.Chem. Soc., 91,4400 (1969) and G. Schwarzenbach, H. Ackerman and R. Ruchstuhl, Helv. Chem. Acta 32,1175 (1949). The mixed aminomethylene carboxylic acids -aminomethylene phosphonic acids may be prepared by reacting primary and secondary amino acids with formaldehyde and phosphorous acid by the method disclosed in U.S. Pat. No. 3,288,846.